Theoretical and experimental comparisons have been made between several nonlinear fracture toughness methods including JIc (J integral method), GCOD (COD method) and GIc wavy bar (nonlinear energy method). Three series of compact tension specimens of 7075-T651, 2124-T851 and Ti-6Al-4V were tested. Five fracture toughness tests, at thicknesses above and below the minimum value for plane strain fracture, were conducted in each series, and toughness values were compared at: (a) the onset of stable crack growth and (b) at the onset of unstable fracture. It was found that when the critical point was the onset of stable crack growth JIc, GIc wave bar and GIc bar (linear toughness) were independent of specimen thickness. When the critical point was the onset of unstable fracture all three toughness values increased with decreasing thickness, with GIc wavy bar JIcGIc bar. The GCOD values were much higher than the others in all cases.